Debian enomalism slow guest network

I have installed debian lenny 64 as host, then installed the enomalism and again debian lenny 64 as guest, everything is cool, but i have noticed that the guest system has very slow network, for example when i copy something via ssh from a computer which is in the same network 100 Mbps it starts at 10 Mb/s and slows down very fast to 100 Kb/s, then in a minute the transfer gets stalled
What can be the problem here, any ideas?

Thanks!

Update: When i copy directly to the host OS, there is no problem it uses 100% capacity of the line....

Update2: Same happens when i try to copy from the host to the guest via scp - the connection gets stalled, and the guest OS seems to not have network connection anymore, when i connect via vnc, everything looks OK, there are also no bad entries in some of the logs, restarting the network fixes the problem, but once the network gets loaded again, the same happens...

The Host system has the following ethernet adapter:
00:07.0 Bridge: nVidia Corporation MCP61 Ethernet (rev a2)

The Guest is detecting (i do not know exactly why, but that was not the real problem):
00:03.0 Ethernet controller: Realtek Semiconductor Co., Ltd. RTL-8139/8139C/8139C+ (rev 20)

The real problem was that the Guest was loading the wrong driver - 3139too, i unloaded it and loaded the 3139cp, and everything is working very fast now. Just copied 1,1GB archive using the 100% of the network capacity!!!